1. Field of the Invention
The present invention relates to a method of manufacturing a surface acoustic wave device used in a high frequency range.
2. Background Information
A surface acoustic wave device propagates surface acoustic waves on the surface of a solid body on which energies are concentrated. Such a device can be miniaturized while still achieving a stable performance. Therefore, such a device is used, for example, as an intermediate frequency filter of a TV receiver or the like. The surface acoustic wave device includes interdigital electrodes provided on a piezoelectric layer. When an AC electric field is applied to the interdigital electrodes on the piezoelectric layer of the surface acoustic wave device having such a structure, strains opposing each other are generated between adjacent electrodes whereby the piezoelectric effect, and surface waves are excited. Bulk single crystal of quartz, LiNbO, LiTaO or the like, and a ZnO thin film vapor deposited on a substrate are used as the materials of the piezoelectric body.
The operation frequency f of the surface acoustic wave device is determined as f=v/.lambda. (v: phase velocity of the acoustic wave propagating along the surface of the solid body, .lambda.: pitch of the interdigital electrodes). More specifically, the device can be used in a higher frequency range if the pitch .lambda. is smaller and the velocity v is larger. However, the velocity v of the surface acoustic wave is limited by the material characteristics of the solid body. Further, there are technical limits of fine processing the electrode width, for example, smallest possible width that can be processed by photolithography is 0.8 .mu.m, while electronic beam exposure techniques enable processing in the order of submicrons. However, the yield becomes lower as the line width becomes thinner, so that there is a lower limit of the pitch of the interdigital electrodes. Accordingly, the highest possible operation frequency of surface acousting wave devices used in practice at present is 900 MHz.
A surface acoustic wave device which can be used in a higher frequency range (GHz range) is desirable, as communications at higher frequencies such as satellite communication and mobile communication have been developed. Diamond has the highest sound velocity among all materials known so far (the velocity of the transverse wave=13000 m/s, the velocity of the longitudinal wave=16000 m/s) and if diamond is used as a base material, the velocity of the surface acoustic wave can be made 10000 m/s or higher. A diamond-like carbon film has a similar sound velocity as diamond, and the velocity v of the surface acoustic wave can be increased to the same extent when a diamond-like carbon film is used as the base material.
Surface acoustic wave devices having a stacked structure of diamond and a piezoelectric body are disclosed, for example, in Japanese Patent Laying-Open Nos. 64-20714, and 64-62911, and the development of such devices has been promoted.
The nature of the piezoelectric layer largely affects the characteristics of the surface acoustic wave device. A specific example of a surface acoustic wave device formed by providing a ZnO film, which is a piezoelectric material, on diamond will be described. Conventionally, the ZnO film has been formed on the diamond by a high frequency sputtering method. In a surface acoustic wave device, the efficiency of excitation of the surface acoustic waves and the propagation loss of the waves are influenced by the c-axis orientation, grain size, surface flatness, resistivity and adhesiveness of the film on the diamond substrate.
If the c-axis orientation of the formed ZnO film is not sufficient, the piezoelectric nature of the ZnO film is not fully exhibited, and the surface acoustic waves are not excited. The smaller .sigma. value of the film in accordance with the X-ray rocking curve analysis of the c-axis orientation is the more preferable. The value must be within 5 degrees, in order to use the device as a surface acoustic wave device. Generally, the .sigma. value of the ZnO film formed on the diamond substrate by RF sputtering method is in the range of 2 to 3 degrees.
The smaller the grain size of the formed ZnO film is, the better the surface flatness, and the smaller becomes the propagation loss of the surface acoustic waves when the surface is more flat. If the propagation loss is too large, the total loss in the device becomes too large, and the device cannot be practically used. To obtain useful devices the surface of the film should be as flat as possible. The ZnO film formed on the diamond substrate by RF sputtering having a .sigma. value of about 2 to 3 degrees would have a film thickness of 1 .mu.m and a grain size of 50 nm, and the corresponding surface roughness is up to about several tens nm.
The piezoelectric film must have a high resistivity to excite the surface acoustic waves. The composition ratio of Zn and O of the ZnO film formed by the RF sputtering method is not 1:1 but an excess of Zn is present. Accordingly, the film turns to be an n type semiconductor, which is conductive. In order to make sure that this film has the required high resistivity, Li has been doped into the ZnO during sputtering to compensate for the charges and to increase the resistivity of the film. The resistivity of the c-axis oriented ZnO film doped with Li during RF sputtering can generally be adjusted at about 10.sup.6 .OMEGA..cm.
As described above, the ZnO polycrystalline film having a superior c-axis orientation formed on a diamond layer by the conventional RF sputtering method has a film thickness of 1 .mu.m and the grain size of the film is about 50 nm. Further, since the ZnO film is doped with Li impurities to provide a higher resistivity, the film has imperfect crystallinity. Therefore, when the surface acoustic wave device is fabricated by using the ZnO film formed by the RF sputtering method, the propagation loss of the surface acoustic waves becomes substantial about 70 dB/cm at 1 GHz. This causes a large insertion loss between the input and output of the device, which leads to a limited application of these devices in practice. The problem of the large propagation loss becomes even more troublesome as the frequency at which the device it is used becomes higher.